It has long been known that exhaust gases produced by combusting hydrocarbon fuels can contribute to atmospheric pollution. Exhaust gases typically contain pollutants such as nitric oxide (NO) and nitrogen dioxide (NO.sub.2), which are frequently grouped together as NO.sub.x, unburned hydrocarbons (UHC), carbon monoxide (CO), and particulates, primarily carbon soot. Nitrogen oxides are of particular concern because of their role in forming ground level smog and acid rain and in depleting stratospheric ozone. NO.sub.x may be formed by several mechanisms. The high temperature reaction of atmospheric oxygen with atmospheric nitrogen, particularly at adiabatic flame temperatures above about 2800.degree. F., forms NO.sub.x through the thermal or the Zeldovich mechanism ("thermal NO.sub.x "). The reaction of atmospheric nitrogen with hydrocarbon fuel fragments (CH.sub.i), particularly under fuel-rich conditions, forms NO.sub.x through the prompt mechanism ("prompt NO.sub.x "). The reaction of nitrogen released from a nitrogen-containing fuel with atmospheric oxygen, particularly under fuel-lean conditions, forms NO.sub.x through the fuel-bound mechanism ("fuel-bound NO.sub.x "). In typical combustors, atmospheric oxygen and nitrogen are readily available in the combustion air which is mixed with the fuel.
While acknowledging a need to control atmospheric pollution, the more advanced combustion control schemes developed during the past decade were designed to maximize combustion efficiency to maintain economic operation with only a secondary regard for pollutant emissions. For example, the production of CO and UHC was considered undesirable, more because it indicated poor combustion efficiency than because CO and UHC are pollutants. To maximize combustion efficiency and flame stability, fuel is often burned in a diffusion flame at fuel/air ratios as near as possible to stoichiometric, that is, at equivalence ratios of slightly less than 1.0. The equivalence ratio is the ratio of the actual fuel/air ratio to the stoichiometric fuel/air ratio. An equivalence ratio of greater than 1.0 indicates fuel-rich conditions, while an equivalence ratio of less than 1.0 indicates fuel-lean conditions. Burning a fuel at an equivalence ratio slightly less than 1.0 produces nearly complete combustion, minimizing CO and UHC production, and a hot flame, maximizing useable energy. The temperatures produced during such an operation are high enough to produce appreciable quantities of thermal NO.sub.x. Therefore, the goal of achieving good thermal efficiency, which arises from economic concerns, is seemingly at odds with the goal of minimizing NO.sub.x emissions, which arises from environmental concerns and is required by increasingly stringent environmental regulations.
Several fairly simple methods are available to decrease NO.sub.x emissions, although none are entirely satisfactory. For example, the formation of fuel-bound NO.sub.x can be minimized or avoided entirely by burninq a low nitrogen or nitrogen-free fuel. However, burning a low nitrogen fuel does nothing to reduce the formation of thermal or prompt NO.sub.x. The formation of thermal NO.sub.x can be reduced by operating under uniformly fuel-lean conditions, such as by using a lean diffusion flame or a lean premixed/prevaporized (LPP) system. The excess air used to achieve fuel-lean combustion acts as a diluent to lower flame temperatures, thereby reducing the amount of thermal NO.sub.x formed. The formation of prompt NO.sub.x can also be reduced by operating under fuel-lean conditions because the excess air decreases the concentration of CH.sub.i available to react with atmospheric nitrogen. However, the extent to which thermal and prompt NO.sub.x formation can be reduced by fuel-lean combustion may be limited by flame instability which occurs at very lean conditions.
Accordingly, what is needed in the art is a method and system for efficiently combusting hydrocarbon fuels while minimizing pollutant emissions, particularly NO.sub.x emissions.